1. Field of the Invention
The present invention relates to an inverted-telephoto type wide angle lens system having a front lens group of a negative refractive power and a rear lens group of a positive refractive power, and more particularly to a focusing thereof.
2. Description of the Prior Art
A wide angle lens used for an interchangeable lens of single lens reflex camera system requires a relatively long back focal length over a predetermined length, in order not to prevent a main mirror of the single lens reflex camera from moving. Therefore, a lens system of an inverted-telephoto type, in which a negative refractive power is located at the object side, is generally applied to the wide angle lens. In such lens system, the entire lens system is normally shiftable in the focusing operation.
However, since such a lens system does not have a symmetrical construction with respect to an aperture diaphragm, it is difficult to maintain a high image reproduction performance when the lens system is focused to a closer object. The reason is that the image is formed on a middle and/or periphery of the image plane. Especially, in the closest focusing condition, the astigmatism will become an over-correcting condition and the spherical aberration will become an under-correcting condition.
To improve the performance a so-called "floating method" has been proposed in which a variable air space is provided in the lens system and the whole of the lens system is shifted while changing the distance of the variable air space in the focusing operation. However, the mechanical construction of the lens becomes more complex if the "floating method" is applied, since a mechanical construction for changing the variable air space in accordance with the shifting of the whole lens system is necessary in addition to another mechanical construction for shifting the whole lens system. Additionally, the whole weight of the lens is increased because of the above mechanical constructions and a plurality of lens barrels. Furthermore, if the focusing operation is performed by a lens driving mechanism of an automatic focusing device provided in the camera, the lens shifting speed will be lowered and the mechanical durability will be deteriorated.